Mixed Calcium Oxide Research Articles

Effect of Different Initial CaO/SiO2 Molar Ratios and Curing Times on the Preparation and Formation Mechanism of Calcium Silicate Hydrate.

To better understand the pozzolanic activity in fly ash used as a supplementary cementitious material in cement or concrete, calcium silicate hydrate (C-S-H) has been synthesized by adding silica fume to a supersaturated calcium hydroxide solution prepared by mixing calcium oxide and ultrapure water. Thermogravimetric analysis results have revealed the variation in the weight loss due to C-S-H in the samples and the conversion ratio of calcium oxide (the μCaO value), which represents the proportion of calcium oxide in the initial reaction mixture used to produce C-S-H, with curing time. The weight loss due to C-S-H and the μCaO value were both maximized (13.5% and 90.4%, respectively) when the initial C/S molar ratio was 1.0 and the curing time was 90 d. X-ray diffraction (XRD) analysis has indicated that C-S-H in the samples after curing for 7 d had the composition Ca1.5SiO3.5·xH2O. 29Si magic angle spinning (MAS) nuclear magnetic resonance (NMR) analysis has revealed that the degree of polymerization of C-S-H increased with an increase in curing time for samples with an initial C/S molar ratio of 1.0. The ratio of internal to terminal tetrahedra (Q2/Q1) increased from 2.29 to 4.28 with the increase in curing time from 7 d to 90 d. At curing times ≥ 28 d, a leaf-like C-S-H structure was observed by scanning electron microscopy (SEM). An ectopic nucleation-polymerization reaction process is proposed for the formation mechanism of C-S-H.

Calcium phosphate powders synthesized from CaCO3 and CaO of natural origin using mechanical activation in different media combined with solid-state interaction.

The highly pure and crystalline calcium carbonate (CaCO3) and calcium oxide (CaO) with small amounts of As, Cd, Hg, and Pb were prepared by calcinating shells of a golden apple snail. Solid-state reaction and mechanical activation between the CaCO3 and CaO from calcined golden apple snail shells and dibasic calcium phosphate dihydrate (CaHPO4•2H2O, DCPD) were performed to develop calcium phosphate powders. The effects of the milling media used on the mechanical activation were examined. A solid-state reaction of manually mixed CaCO3 or CaO with DCPD powders at a temperature of 1100°C produced mostly β-tricalcium phosphate (β-TCP). Hydroxyapatite (HAp) with a small quantity of β-TCP could be produced from a mixed CaCO3+DCPD powder using dry and wet mechanical activations with distilled water, alcohol and acetone and from a mixed CaO+DCPD powder using dry mechanical activation combined with a solid-state reaction at a temperature of 1100°C. A phase change of milled powders to β-TCP was clearly observed from a wet mechanical activation of CaO+DCPD powder with distilled water or alcohol in a solid-state reaction. The thermal instability of HAp powders from a combined mechanical activation with solid-state reaction of CaCO3 or CaO and DCPD powders could result from two factors. The first is that the pollution was released from the balls and pot mill materials during the mechanical process. Another factor is a reduced level of calcium in the CaO+DCPD mixed powder due to a reaction between CaO and water or alcohol during mechanical milling.

Preparation of Calcium Polysulfide to Remediate Groundwater Contaminated by Hexavalent Chromium

Polysulfide has been verified to exhibit a strong ability in reducing hexavalent chromium (Cr(VI)). This study prepared CPS by using calcium oxide and sulfur powder to investigate the removal efficiency of Cr(VI) by the self-prepared CPS and the effects of pH and dissolved oxygen of groundwater on the reduction of Cr(VI) by CPS. The results show that the CPS produced by mixing calcium oxide, sulfur powder, and distilled water at a ratio of 1:2:10 and heating the mixture for 60 min can effectively reduce the Cr(VI) in groundwater. This method can be provided to produce the agents required for contamination remediation, which can reduce the monetary cost on purchasing the agents and the amount of carbon emission resulting from long-distance transportation, achieving the goal of green remediation. CPS prepared in this study, the removal rate of Cr(VI) can reach more than 90% when application rate was 200 at sample/CPS volume ratio.

Insight into the atomic scale structure of CaF2-CaO-SiO2 glasses using a combination of neutron diffraction, 29Si solid state NMR, high energy X-ray diffraction, FTIR, and XPS

Abstract Bioactive glasses are important for biomedical and dental applications. The controlled release of key ions, which elicit favourable biological responses, is known to be the first key step in the bioactivity of these materials. Properties such as bioactivity and solubility can be tailored for specific applications. The addition of fluoride ions is particularly interesting for dental applications as it promotes the formation of fluoro-apatite. To date there have been mixed reports in the literature on how fluorine is structurally incorporated into bioactive glasses. To optimize the design and subsequent bioactivity of these glasses, it is important to understand the connections between the glass composition, structure and relevant macroscopic properties such as apatite formation and glass degradation in aqueous media. Using neutron diffraction, high energy X-ray diffraction, 29Si NMR, FTIR and XPS we have investigated the atomic scale structure of mixed calcium oxide / calcium fluoride silicate based bioactive glasses. No evidence of direct Si-F bonding was observed, instead fluorine was found to bond directly to calcium resulting in mixed oxygen/fluoride polyhedra. It was therefore concluded that the addition of fluorine does not depolymerise the silicate network and that the widely used network connectivity models are valid in these oxyfluoride systems.

Estimating fracture toughness of C–S–H using nanoindentation and the extended finite element method

In this paper, an approach to integrating nanoindentation testing and finite element simulations is introduced to compute the fracture toughness of cementitious materials. Calcium silicate hydrate (C–S–H) was synthesized using the standard procedure of mixing calcium oxide (CaO) and silicate (SiO2) at a mixture ratio of 1.5. C–S–H powder was filtered, dried to a relative humidity of 11%, and then compacted at 400 MPa. Nanoindentation tests incorporating dwell time were performed on polished C–S–H specimens using a Berkovich indenter tip. The reduced elastic moduli of the C–S–H specimens were extracted from the nanoindentation measurements. Viscoelastic and viscoelastic-plastic finite element models with creep and cracking capabilities were developed to simulate the nanoindentation tests and to extract the fracture energy. The viscoelastic-plastic model utilized the extended finite element method (XFEM) to describe cracking and evaluate the cracking surface of C–S–H. The analysis showed that the proposed approach could fairly predict the fracture energy release rate and thus fracture toughness of C–S–H. The calculated fracture toughness was in agreement with the fracture toughness values reported in the literature.

Carbonation and deactivation kinetics of a mixed calcium oxide–copper oxide sorbent/oxygen carrier for post-combustion carbon dioxide capture

Removing carbon dioxide from industrial effluents via solid carbonate sorbents is a potential greenhouse gas mitigation strategy that can also produce hydrogen from the effluent in water gas shift reactors downstream. However, to regenerate the calcium oxide (from the carbonate) requires high temperature that is provided via an oxidation step. To avoid costly oxycombustion to regenerate the CaCO3, it is possible to use the heat generated during the reduction of copper oxide and manganese oxides as chemical looping agents. Unfortunately, sorbents sinter thereby and consequently the surface area drops in the regeneration step. A mixed CuO–CaO sorbent with 10% calcium aluminate binder lost 84% of its initial BET surface area during the first 100min on stream. Homogeneous calcium–copper–cement oxide sorbent deactivate to the 0.5 order with respect to the remaining surface area and a decay constant equal to 2.86min−1. An empirical equilibrium power law model characterizes the carbonation rate from 425°C to 665°C with 10% to 20% CO2 in a 45mm fixed bed. The apparent activation energies of the forward and reverse reaction are 220 and 120kJmol−1, respectively. In addition to sintering, the reaction rate of the regeneration step decreases with time on stream following a zero order process. We attributed this gradual irreversibility to increased crystallinity, reducing the diffusion rate of CO2 to the active carbonate phase. By integrating this resistance to the kinetic model, we are able to explain 99% of the variance in the gas profiles measured by a mass spectrometer.

Calcium oxide based materials for thermochemical heat storage in concentrated solar power plants

The present study relates to the preparation of mixed calcium oxide–alumina compositions as candidate materials for a cyclic thermochemical hydration–dehydration scheme at moderate to high temperatures (e.g. 400–600°C) that can offer the possibility of short and long term energy storage, particularly suitable for concentrated solar power installations. The synthesized materials were assessed in terms of their cyclic hydration–dehydration performance in the temperature range of 200–550°C. Acknowledging the fact that the particular thermochemical scheme has been identified to result in substantial cycle-to-cycle fragmentation of pure CaO particles which is detrimental to particle reactor bed concepts, one of the main purposes of using Al as additive is related to materials structural enhancement. To this respect, preliminary studies related to macro-structural integrity assessment were also conducted. In addition, the performance of synthesized material is compared to the one of natural lime (benchmark material). The additive content spanned over a wide range of Ca/Al molar ratios, namely from 95/5 to 52/48, while two different calcium oxide precursors, i.e. calcium nitrate and calcium acetate, were employed. Fresh and hydrated compositions were characterized in-detail with respect to their physicochemical properties in order to correlate different behaviors with certain attributes of the materials. Synthetic materials, both calcium nitrate and calcium acetate derived, favored the formation of Ca/Al mixed phases. The latter led to materials with higher surface areas and, for a given Ca/Al ratio, resulted to higher hydration/dehydration performance. Mixed oxides, although capable of being hydrated at ambient temperature, did not participate in the reaction scheme at temperatures ⩾200°C and thus presence of such phases resulted in considerable decrease of hydration/dehydration capacity versus the one of natural lime. On the other hand, the presence of such mixed compositions improved, albeit not dramatically, macro-structural integrity. A relatively good combination of hydration–dehydration performance with better-than-natural lime structural integrity was achieved for the mixed materials with a Ca/Al molar ratio equal to 89/11 and 81/19 molar ratio. The Ca-precursor used in these materials slightly affected their cyclic performance with the ex-CaN ones presenting better behavior.

Steam–Coal Gasification Using CaO and KOH for in Situ Carbon and Sulfur Capture

We present experimental results of coal gasification with and without the addition of calcium oxide and potassium hydroxide as dual-functioning catalyst–capture agents. Using two different coal types and temperatures between 700 and 900 °C, we studied the effect of these catalyst–capture agents on (1) the syngas composition, (2) CO2 and H2S capture, and (3) the steam–coal gasification kinetic rate. The syngas composition from the gasifier was roughly 20% methane, 70% hydrogen, and 10% other species when a CaO/C molar ratio of 0.5 was added. We demonstrated significantly enhanced steam–coal gasification kinetic rates when adding small amounts of potassium hydroxide to coal when operating a CaO–CaCO3 chemical looping gasification reactor. For example, the steam–coal gasification kinetic rate increased 250% when dry mixing calcium oxide at a Ca/C molar ratio of 0.5 with a sub-bituminous coal, and the kinetic rate increased 1000% when aqueously mixing calcium oxide at a Ca/C molar ratio of 0.5 along with potassium hydroxide at a K/C molar ratio of 0.06. In addition, we conducted multi-cycle studies in which CaCO3 was calcined by heating to 900 °C to regenerate the CaO, which was then reused in repeated CaO–CaCO3 cycles. The increased steam–coal gasification kinetics rates for both CaO and CaO + KOH persisted even when the material was reused in six cycles of gasification and calcination. The ability of CaO to capture carbon dioxide decreased roughly 2–4% per CaO–CaCO3 cycle. We also discuss an important application of this combined gasifier–calciner to electricity generation and selling the purge stream as a precalcined feedstock to a cement kiln. In this scenario, the amount of purge stream required is fixed not by the degradation in the capture ability but rather by the requirements at the cement kiln on the amount of CaSO4 and ash in the precalcined feedstock.

Catalysts derived from waste sources in the production of biodiesel using waste cooking oil

Catalysts fully derived from waste materials in order to make sustainable chemical reactions have been a recent topic of interest. Calcined (900°C, 2h) calcium oxide (CaO) sourced from waste mud crab shells and cockleshells were mixed in a 1:1 mass ratio to be used for transesterification of waste cooking oil (WCO) at 5wt.% catalyst and a methanol to oil molar ratio of 13:1 for 3h at methanol refluxing temperature to give 98% conversion. Then, boiler ash (BA) from agricultural waste was used to transesterify WCO at 3wt.% catalyst and a methanol to oil molar ratio of 15:1 for 0.5h at methanol refluxing temperature to give 89% conversion. In order to reduce the reaction time (for CaO-catalyzed reaction) and to increase the conversion (for BA-catalyzed reaction) various amounts of BA were added to 5wt.% of mixed CaO and optimized using Central Composite Design (CCD). The optimized conditions predicted by CCD were found to be 3.39wt.% boiler ash, with a reaction time of 32.3min and a conversion of 99.5%. The predicted results agree with the experimental results (3wt.% boiler ash, reaction time of 30min with 99% conversion).

MgAl2O4 Spinel-Stabilized Calcium Oxide Absorbents with Improved Durability for High-Temperature CO2 Capture

With efficient energy recovery, calcium-oxide-based absorbents that operate at elevated temperatures have an advantage over absorbents that operate at lower temperatures for CO2 capture from coal power plants. The major limitation of these absorbents is that the carbonation and decarbonation reactions of CaO and CaCO3 are far from complete or reversible. Rapid loss of CO2 capacity over many carbonation/decarbonation cycles is always observed because of severe absorbent sintering. We have found that this sintering effect can be effectively mitigated by properly mixing calcium oxide precursors with small rod-like MgAl2O4 spinel nanoparticles. A new class of CaO-based absorbents with much improved high-temperature durability was developed by wet physical mixing of calcium acetate with nano MgAl2O4 spinel particles followed by high-temperature calcination. CaO−MgAl2O4 (32 wt % spinel content) material provides 34 wt % CO2 capacity after 65 carbonation−decarbonation cycles (650 and 850 °C, respectively), corre...

생석회의 팽창압 발현에 미치는 첨가제 및 양생온도의 영향

Calcium oxide has been used as a demolition agent in fracturing rocks and old concrete structures, etc. With the agent, demolition work can be done in safety without a noise, vibration and any other pollution, since high expansive pressure is obtained gradually by only mixing the agents with water and pouring the slurry into boreholes. But application of the non-explosive demolition agent is a time-consuming job, especially in winter. Essentially, this problem is related to the reaction rate of calcium oxide with water. This study examines the influence of additives such as cement and anhydrite on expansion pressure of calcium oxide at different curing temperatures. The expansion pressure of calcium oxide began to increase steadily with the rise of the curing temperature. When mixing calcium oxide alone with water, blown-out shot occurred. But as additives were added to calcium oxide, the reaction of calcium oxide delayed and the expansion pressure showed gradual increment. Especially, anhydrite showed a superior delaying effect than cement on the reaction of calcium oxide.